basalt

A hard, fine-grained, dark-colored rock of volcanic origin
composed primarily of plagioclase feldspar,
and pyroxene, together with other minerals,
usually including olivine and ilmenite
(an oxide of iron and titanium). It contains less than about 52 percent
by weight of silica (SiO2), which
results in a low viscosity (resistance
to flow.)

Because of basalt's low viscosity, basaltic lava
can flow quickly and easily move more than 20 km from a vent. The low viscosity
typically allows volcanic gases to escape without generating enormous eruption
columns. Basaltic lava fountains and
fissure eruptions, however, still form explosive fountains hundreds of meters
tall. Basalt is erupted at temperatures between 1100° to 1250°C.

Basalt is the most common extrusiveigneous
rock on the terrestrial planets and is the most common rock type in the
Earth's crust (the outer 10 to 50 km). It
covers about 70% of Earth's surface, including most of the ocean floor.

Huge outpourings of lava called flood basalts are found
on many continents. For example, the Columbia River basalts, which erupted
15 to 17 million years ago, cover most of southeastern Washington and regions
of adjacent Oregon and Idaho. It can form vast plateaus, such as the 200,000-square-mile
Deccan of India. Basalt can also assume a striking columnar structure, as
exhibited in the Palisades along the Hudon River, the Devil's Postpile in
California, and the Giant's Causeway
in Ireland.

Basaltic magma is commonly produced by direct
melting of the Earth's mantle, the region
of the Earth below the outer crust. On continents, the mantle begins at
depths of 30 to 50 km.

Shield volcanoes, such as those that
make up the islands of Hawai'i, are composed almost entirely of basalt as,
indeed, are most oceanic islands.

Uses of basalt

Basalt was widely used by the ancient Egyptians for their sculpture, and
in the construction of tombs and obelisks. The name is also applied to a
pottery of basalic appeaance made by Josiah Wedgewood.